Neutron Generator Technology

PNL’s neutron generator technology utilizes the DD and DT nuclear fusion reactions to generate neutrons. There are many commercially available DD and DT neutron generators but most of them operate solid targets that limit the lifetime and neutron yield of these devices. Furthermore, common COTS solutions employ ion sources that fundamentally limit the amount of beam current that can be accelerated for neutron production.

PNL’s solution utilizes a microwave ion source (MWS) and a gaseous deuterium, tritium, or mixed species target, which allows for a significantly increased neutron yield, as well as longer target and source lifetimes for up to several years. A high voltage DC power supply that was designed internally at PNL, brings the entire top sheet, and all electronics on it, to a potential of 300 kV.  A magnetron produces microwaves that are guided into a plasma chamber filled with deuterium gas. An 875 Gauss magnetic field is generated within the plasma chamber, and the interaction of the 2.45 GHz microwaves in the presence of the specific magnetic field configuration, causes the gas to be ionized in a very efficient manner. D+ ions are then extracted from the plasma chamber and accelerated to 300 kV by a direct-inject, CW accelerator. The beam is then focused by a magnetic solenoid and transported down a beam line with several stages of differential pumping, until it impinges upon a target chamber filled with deuterium and/or tritium gas, where nuclear fusion occurs and then neutrons are produced.

A schematic and image of the PNL neutron generator is shown below:

There are no windows separating the target chamber from the accelerating beam line because any window material would either be so thick that it would completely stop the incident ion beam or, it would be instantly melted by the 15 kW of beam power. In order for the target gas to stop the ion beam and produce neutrons within a reasonable distance (e.g. < 1 meter), the target pressure must be in the Torr pressure regime. However, in order to effectively transport the ion beam, the acceleration region of the beam line must be kept at a pressure approximately one million times lower than this. In order to achieve this pressure differential, a series of differential pumping stages are employed.

Currently, PNL’s generator can operate in a steady state mode at deuterium currents up to 50 mA. Measured neutron yields have exceeded 3×1011 DD (2.5 MeV) neutrons per second. Replacing the target gas with tritium does not alter the operation of the generator but results in an increase in neutron yield of ~100X (to over 3×1013 DT n/s) and increases the neutron energy to 14.1 MeV.